/dev/sdb got mounted root, 'guess' algorithm exludes /dev/sda (among others)

Full unit log: http://paste.ubuntu.com/7898450/

for some reason on a juju (1.20.2 prerelease) + maas (1.6.0+bzr2539-0ubuntu1~beta6~ppa1) deploy, I got /dev/sdb as my root device. swift-storage didn't like this.

/dev/sda was free and could have been used, but the charm didn't like this in it's 'guess' algorithm.

ubuntu@bohr:~$ tail -30 /var/log/juju/unit-swift-storage-0.log
2014-07-29 20:29:34 INFO install GPT data structures destroyed! You may now partition the disk using fdisk or
2014-07-29 20:29:34 INFO install other utilities.
2014-07-29 20:29:34 INFO install Warning: The kernel is still using the old partition table.
2014-07-29 20:29:34 INFO install The new table will be used at the next reboot.
2014-07-29 20:29:34 INFO install The operation has completed successfully.
2014-07-29 20:29:34 INFO install 1+0 records in
2014-07-29 20:29:34 INFO install 1+0 records out
2014-07-29 20:29:34 INFO install 1048576 bytes (1.0 MB) copied, 0.00211706 s, 495 MB/s
2014-07-29 20:29:34 INFO install 100+0 records in
2014-07-29 20:29:34 INFO install 100+0 records out
2014-07-29 20:29:34 INFO install 51200 bytes (51 kB) copied, 0.00456763 s, 11.2 MB/s
2014-07-29 20:29:34 INFO install mkfs.xfs: cannot open /dev/sdb: Device or resource busy
2014-07-29 20:29:34 INFO install <open file '/proc/partitions', mode 'r' at 0x7ffedc45fdb0>
2014-07-29 20:29:34 INFO install Traceback (most recent call last):
2014-07-29 20:29:34 INFO install File "/var/lib/juju/agents/unit-swift-storage-0/charm/hooks/install", line 91, in <module>
2014-07-29 20:29:34 INFO install main()
2014-07-29 20:29:34 INFO install File "/var/lib/juju/agents/unit-swift-storage-0/charm/hooks/install", line 85, in main
2014-07-29 20:29:34 INFO install hooks.execute(sys.argv)
2014-07-29 20:29:34 INFO install File "/var/lib/juju/agents/unit-swift-storage-0/charm/hooks/charmhelpers/core/hookenv.py", line 478, in execute
2014-07-29 20:29:34 INFO install self._hooks[hook_name]()
2014-07-29 20:29:34 INFO install File "/var/lib/juju/agents/unit-swift-storage-0/charm/hooks/install", line 45, in install
2014-07-29 20:29:34 INFO install setup_storage()
2014-07-29 20:29:34 INFO install File "/var/lib/juju/agents/unit-swift-storage-0/charm/hooks/swift_storage_utils.py", line 180, in setup_storage
2014-07-29 20:29:34 INFO install mkfs_xfs(dev)
2014-07-29 20:29:34 INFO install File "/var/lib/juju/agents/unit-swift-storage-0/charm/hooks/swift_storage_utils.py", line 172, in mkfs_xfs
2014-07-29 20:29:34 INFO install check_call(cmd)
2014-07-29 20:29:34 INFO install File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
2014-07-29 20:29:34 INFO install raise CalledProcessError(retcode, cmd)
2014-07-29 20:29:34 INFO install subprocess.CalledProcessError: Command '['mkfs.xfs', '-f', '-i', 'size=1024', '/dev/sdb']' returned non-zero exit status 1
2014-07-29 20:29:34 ERROR juju.worker.uniter uniter.go:486 hook failed: exit status 1
ubuntu@bohr:~$ ll /dev/sd*
brw-rw---- 1 root disk 8, 0 Jul 29 20:27 /dev/sda
brw-rw---- 1 root disk 8, 1 Jul 29 20:27 /dev/sda1
brw-rw---- 1 root disk 8, 2 Jul 29 20:27 /dev/sda2
brw-rw---- 1 root disk 8, 16 Jul 29 20:29 /dev/sdb
brw-rw---- 1 root disk 8, 17 Jul 29 20:27 /dev/sdb1
ubuntu@bohr:~$ ll /proc/partitions
-r--r--r-- 1 root root 0 Jul 29 20:34 /proc/partitions
ubuntu@bohr:~$ cat /proc/partitions
major minor #blocks name

   8 0 119454720 sda
   8 1 118405103 sda1
   8 2 1047552 sda2
   8 16 117220824 sdb
   8 17 117219800 sdb1

# /dev/sdb is root (!)
# The landscape cloud-installer tells the charm to use sd[a-z] more or less.

ubuntu@bohr:~$ mount |grep sdb
/dev/sdb1 on / type ext4 (rw)
ubuntu@bohr:~$ mount |grep sda
ubuntu@bohr:~$ ll /dev/sda*
brw-rw---- 1 root disk 8, 0 Jul 29 20:27 /dev/sda
brw-rw---- 1 root disk 8, 1 Jul 29 20:27 /dev/sda1
brw-rw---- 1 root disk 8, 2 Jul 29 20:27 /dev/sda2